The present invention relates to an electromagnetic motor, for example, for use in a timepiece.
The invention is concerned particularly with a motor comprising a stator formed of two pole-pieces between which there is provided an opening, the pole-pieces being interconnected by an element of high magnetic permeability around which is wound a coil, and a rotor mounted for rotation within the said opening and having a shaft and a bar magnet which is magnetized in the direction of its axis. The sum (W) of the widths (w.sub.1, w.sub.2) of the clearances between the stator and the poles of the bar magnet is a periodic function W(.theta.) of the angular position (.theta.) of the bar magnet, which periodic function, during each period, passes through a minimum value and a maximum value at a predetermined first angular position (.theta.m) and a predetermined second angular position (.theta.M) respectively of the rotor.
In such motors, in the absence of any driving pulse applied to the coil, the the rotor is subjected to a positioning torque the value of which is a function of the variable .theta.. In the absence of any driving pulse, this positioning torque causes the rotor to assume a rest position (.theta.m) corresponding substantially to the value of .theta. for which the function W(.theta.) has its minimum value.
In Belgian patent specification No. 550,943 there is disclosed an electromagnetic motor of the type referred to above, in which the rest position of the rotor is determined by two projecting parts provided on the wall surfaces which delimit the opening within which the rotor is located. This known motor is illustrated, diagrammatically and partly broken away, in FIG. 1 of the accompanying drawings. It can be seen from this figure that the motor comprises a circular section rotor 1 in which is incorporated a diametrically extending bar magnet 2. The stator consists of two pole pices 3a, 3b between which there is provided an opening 4. The opening 4 is delimited by two oppositely located wall surface parts 5 and 6. These wall surface parts 5 and 6 are interconnected by two zones of high magnetic reluctance which consist in this case of air gaps or clearances 7 and 8. The rest position of the rotor 1, which is shown in FIG. 1, is determined by two projections 5a, 6a provided respectively on the two wall surface parts 5, 6. In this known motor, the projections 5a, 6a are delimited by steeply inclined wall parts 9, 10. Consequently, the positioning torque which is normally applied to the rotor has, in the absence of any driving pulse, a relatively small value within a relatively wide range of values between the values .theta.m and .theta.M.
According to other prior art, which is the subject of Swiss patent application No. 619,107, and is illustrated in FIG. 2 of the accompanying drawings, the stator of an electromagnetic motor is made in one piece with narrow connecting parts 11, 12 constituting, in known manner, zones of high magnetic reluctance between the two pole-pieces 3a, 3b of the stator. The opening 4 between the pole-pieces 3a, 3b is of generally cylindrical shape, being of circular cross-section except in two diametrically opposite regions where it is formed with flats 13a, 13b. This known motor has a rotor 14 of circular section which is composed entirely of magnetizable material and is magnetized in the direction of a diametral axis. Alternatively the rotor 14 could be magnetized only in an outer annular region thereof.
In this known motor, because of the circular-section shape of the magnet, the positioning torque applied to the rotor is a sinusoidal function of the angular position of the latter. Due to the characteristic form of the sinusoidal function, the amplitude of this positioning torque only attains substantial values when the rotor is in angular positions which are comparatively remote from the rest position of the latter. As a result, in order to ensure reliable starting of the rotor each time a driving pulse is applied to the coil, it is necessary that these driving pulses should be of relatively large amplitude. This results in an increase in the consumption of electric current by the motor.
An important object of the present invention is to obviate these disadvantages.